Old-growth forest

Old-growth forest
Old-growth European Beech forest in Biogradska Gora National Park, Montenegro
Coast Redwoods in old-growth forest in Muir Woods National Monument, Marin County, California.

An old-growth forest (also termed primary forest, virgin forest, primeval forest, late seral forest, or in Britain, ancient woodland) is a forest that has attained great age (and associated structural features), and thereby exhibits unique ecological features.[1]



Old-growth forests tend to have more large trees and standing dead trees, multi-layered canopies with gaps resulting from the deaths of individual trees, and coarse woody debris on the forest floor.[2]

Forest regenerated after a severe disruption, such as wildfire, insect infestations or harvesting, is often called second-growth or regeneration until enough time passes for the effects of the disturbance to be no longer evident. Depending on the forest, this may take anywhere from a century to several millennia. Hardwood forests of the eastern United States can develop old-growth characteristics in one or two generations of trees, or 150–500 years. In British Columbia, Canada, old growth is defined as 120 or 140 years of age in the interior of the province where fire is a frequent and natural occurrence. In British Columbia’s coastal rainforests, old growth is defined as trees more than 250 years, with some trees reaching more than 1,000 years of age.[3]

Forest types have very different development patterns, natural disturbances and appearances. A Douglas-fir stand may grow for centuries without disturbance while an old-growth ponderosa pine forest requires frequent surface fires to reduce the shade-tolerant species and regenerate the canopy species.[4] In the boreal forest, catastrophic disturbances such as wildfire minimize opportunities for major accumulations of dead and downed woody material and other structural legacies associated with old growth conditions.[5]

Many botanists specifically define old growth in terms of meeting several criteria, under which system forests with sufficient age and minimal disturbance are considered old growth. Typical characteristics of old-growth forest include presence of older trees, minimal signs of human disturbance, mixed-age stands, presence of canopy openings due to tree falls, pit-and-mound topography, down wood in various stages of decay, standing snags (dead trees), multi-layered canopies, intact soils, a healthy fungal ecosystem, and presence of indicator species.


The Northern Spotted Owl primarily inhabits old growth forests in the northern part of its range (Canada to southern Oregon) and landscapes with a mix of old and younger forest types in the southern part of its range (Klamath region and California).

Old-growth forests are often biologically diverse, and home to rare species, threatened species, and endangered species of plants and animals, such as the northern spotted owl, marbled murrelet and fisher, making them ecologically significant. Levels of biodiversity may be higher or lower in old-growth forests compared to that in second-growth forests, depending on specific circumstances, environmental variables and geographic variables. Logging in old-growth forests is a contentious issue in many parts of the world. Excessive logging can reduce biodiversity, affecting not only the old-growth forest itself, but also indigenous species that rely upon old-growth forest habitat.[6][7]

Mixed age

A forest in old-growth stage has a mix of tree ages, due to a distinct regeneration pattern for this stage. New trees regenerate at different times from each other, because each one of them has different spatial location relative to the main canopy and hence each one receives a different amount of light. The mixed age of the forest is an important criterion in ensuring that the forest is a relatively stable ecosystem in the long term. A climax stand that is a uniformly-aged becomes senescent and degrades within a relatively short time-period to result in a new cycle of forest succession. Thus, it is a less stable ecosystem.

Canopy openings

Forest canopy gaps are essential in creating and maintaining mixed-age stands. Also, some herbaceous plants only become established in canopy openings, but persist beneath an understory. Openings are a result of tree death due to small impact disturbances such as wind, low-intensity fires and tree diseases.

Old-growth forests are unique, usually having multiple horizontal layers of vegetation representing a variety of tree species, age classes, and sizes, as well as "pit and mound" soil shape with well-established fungal nets.[8] Because old-growth forest is structurally diverse it provides higher-diversity habitat than forests in other stages. Thus, sometimes higher biological diversity can be sustained in old-growth forest, or at least a biodiversity that is different from other forest stages.


The characteristic topography of much old-growth forest consists of pits and mounds. Mounds are caused by decaying fallen trees, and pits (tree throws) by the roots pulled out of the ground when trees fall due to natural causes, including being pushed over by animals. Pits expose humus-poor, mineral-rich soil and often collect moisture and fallen leaves, forming a thick organic layer that is able to nurture certain types of organisms. Mounds provide a place free of leaf inundation and saturation, where other types of organisms thrive.

Standing snags

Standing snags provide food sources and habitat for many types of organisms. In particular, many species of dead-wood predators such as woodpeckers must have standing snags available for feeding. In North America the spotted owl is well known for needing standing snags for nesting habitat.

Decaying ground layer

Downed wood replenishes topsoil as it decays
Fungus on a tree stump in the Białowieża Forest, one of the last largely intact primeval forests in Central Europe

Fallen timber, or coarse woody debris, contributes carbon-rich organic matter directly to the soil, thus providing a substrate for mosses, fungi and for seedlings, and in creating microhabitats by creating relief on the forest floor. In some ecosystems, such as the temperate rain forest of the North American Pacific coast, fallen timber may become nurse logs, providing a substrate for seedling trees.


Intact soils harbor many life-forms that rely on them. Intact soils generally have very well-defined horizons, or soil profiles. Different organisms may need certain well-defined soil horizons in order to live, while many trees need well-structured soils free of disturbance in order to thrive. Some herbaceous plants in northern hardwood forests must have thick duff layers (which are part of the soil profile). Fungal ecosystems are essential for efficient in-situ recycling of nutrients back into the entire ecosystem.


Ecological definitions

Stand age definition

Stand age can also be used to categorize forest as old-growth.[9] For each geographical area, there is an average time since disturbance when the forest will reach old-growth stage. This method is useful, because it allows quick and objective determination of forest stage. However, this definition does not provide explanation about forest function. It just gives a useful number to measure. Due to that fact, some forests may be excluded from being categorized as old-growth even if they have old-growth attributes just because they are too young. Also, older forests can lack some old-growth attributes and be categorized as old-growth just because they are so old. The idea of using age is also problematic, because human activities can influence the forest in varied ways. For example, after logging of 30% of the trees, we can wait less time for old-growth to come back than after removal of 80% of the trees.

Forest dynamics definition

From a forest dynamics perspective, old-growth forest is a forest in a stage that follows Understory Reinitiation stage.[10] A review of the stages helps to understand the concept:

  1. Stand-replacing: disturbance hits the forest and kills most of the living trees.
  2. Stand-initiation: population of new trees becomes established.
  3. Stem-exclusion: trees grow higher and enlarge their canopy, thus competing for the light with neighbors. Light competition mortality kills slowly growing trees and reduces forest density. This allows surviving trees to increase in size. Eventually the canopies of neighboring trees touch each other and drastically lowers amount of light that reaches lower layers. Due to that, the understory dies and only very shade-tolerant species survive.
  4. Understory reinitiation: trees die from low level mortality, such as windthrow and diseases. Individual canopy gaps start to appear and more light can reach forest floor. Hence, shade-tolerant species can establish in the understory.
  5. Old-growth: Main canopy trees become older and more of them die, creating even more gaps. Since the gaps appear at different times, the understory trees will be at different growth stages. Furthermore, the amount of light that reaches each understory tree depends on its position relative to the gap. Thus, each understory tree grows at a different speed. The difference in establishment timing and in growth speed create a population of understory trees that are variable in size. Eventually, some understory trees grow to become as tall as the main canopy trees, thereby filling the gap. This perpetuation process is typical for the old-growth stage. This, however, does not mean that the forest will be old-growth forever. Generally there are three possible futures for old-growth stage forest: (A) The forest will be hit by a new stand-replacing disturbance and most of the trees will die. (B) The tree community will eventually create unfavorable conditions for new trees to regenerate. In this case, the old trees will die and smaller plants will create woodland. (C) The regenerating understory trees are different species than the main canopy trees. In this case, the forest will switch back to Stem-Exclusion stage, but with different tree species. The forest in old-growth stage can be stable for centuries but the length of this stage depends on the forest's tree composition and climate of the area. For example, frequent natural fires do not allow boreal forests to be as old as coastal forests of western North America.

Of importance is that while the stand switches from one tree community to another, the stand will not necessarily go through old-growth stage between those stages. Some tree species have relatively open canopy. That allows more shade-tolerant tree species to establish below even before Understory Reinitiation stage. The shade-tolerant trees will eventually out-compete the main canopy trees in stem-exclusion stage. Therefore, the dominant tree species will change, but the forest will still be in Stem-Exclusion stage.

Tree species succession may change tree species composition once the old-growth stage has been achieved. For example, an old boreal forest may contain some large aspen trees, which may die and be replaced by smaller balsam fir or black spruce. Consequently, the forest will switch back to Understory Reinitiation stage.[11] If old growth stage is seen as an end point of stand development, it can be easily evaluated using structural or static attributes. However, in some forest ecosystems this can lead to decisions regarding the preservation of unique stands or attributes that will disappear over the next few decades because of natural succession processes. Consequently, using stand dynamics to define old-growth forest is more useful in cases where the species that constitute old-growth forest can have long life span or in ecosystem where succession is very slow.

Social and cultural definitions

Redwood tree in northern California redwood forest. According to the National Park Service "96 percent of the original old-growth coast redwoods have been logged."[12]

Common cultural definitions and common denominators regarding what comprises old-growth forest, and of the variables that define, constitute and embody old-growth forests include:

  • The forest habitat possesses relatively mature, old trees;
  • The old-growth trees have long continuity on the same site;
  • The forest itself has not been subjected to significant inhabitation by mankind that has altered the appearance of the landscape and its ecosystems, has not been subjected to logging, and has inherently progressed per natural tendencies.

The debate over old growth definitions has been inextricably linked with a complex range of social perceptions about wilderness preservation, aesthetics and spirituality, as well as economic or industrial values.[13]

Economic definitions

Old-growth forests were often given harvesting priority because they have the most commercially valuable timber, they are considered to be at greater risk of deterioration through root rot or insect infestation, and they occupy land that could be used for more productive second-growth stands.[14] In some regions, old growth is not the most commercially viable timber – in British Columbia, Canada, harvesting in the coastal region is moving to younger second-growth stands.[15]

Other definitions

A 2001 scientific symposium in Canada found that defining old growth in a scientifically meaningful, yet policy-relevant, manner presents some basic difficulties, especially if a simple, unambiguous, and rigorous scientific definition is sought. Symposium participants identified some attributes of late-successional, temperate-zone, old-growth forest types that could be considered in developing an index of "old-growthness" and for defining old-growth forests:[16]

Structural features:

Avatar Grove near Port Renfrew, British Columbia. Giant Douglas-firs (left) and red cedars (right) fill the grove.
  • Uneven or multi-aged stand structure, or several identifiable age cohorts
  • Average age of dominant species approaching half the maximum longevity for species (approximately 150+ years for most shade-tolerant trees)
  • Some old trees at close to their maximum longevity (ages of 300+ years)
  • Presence of standing dead and dying trees in various stages of decay
  • Fallen, coarse woody debris
  • Natural regeneration of dominant tree species within canopy gaps or on decaying logs

Compositional features:

  • Long-lived, shade-tolerant tree species associations (e.g., sugar maple, American beech, yellow birch, red spruce, eastern hemlock, white pine)

Process features:

  • Characterized by small-scale disturbances creating gaps in forest canopy
  • A long natural rotation for catastrophic or stand-replacing disturbance (e.g., a period greater than the maximum longevity of the dominant tree species)
  • Minimal evidence of human disturbance
  • Final stages of stand development before a relatively steady state is reached


  • Old-growth forests often contain rich communities of plants and animals within the habitat due to the long period of forest stability. These varied and sometimes rare species may depend on the unique environmental conditions created by these forests.
  • Old-growth forest serves as a reservoir for species which cannot thrive or easily regenerate in younger forest, and so can be used as a baseline for research.
  • Plant species that are native to old-growth forests may someday prove to be invaluable towards curing various human ailments, as has been realized in numerous plants in tropical rainforests.[17][18]
  • Old-growth forests also store large amounts of carbon above and below the ground (either as humus, or in wet soils as peat). They collectively represent a very significant store of carbon. Destruction of these forests releases this carbon as greenhouse gases, and may increase the risk of global climate change.[19]

Ecosystem services

Old-growth forests provide ecosystem services that may be far more important to society than their use as a source of raw materials. These services include breathable air, pure water, carbon storage, regeneration of nutrients, maintenance of soils, pest control by insectivorous bats and insects, micro- and macro-climate control, and the storage of a wide variety of genes.[20]

Climatic impacts

Old-growth forests store large amounts of carbon in wood, humus, and peat. [19]

Old-growth forests are an important part of carbon sequestration and its impacts on climate change and climate change mitigation. The Intergovernmental Panel on Climate Change said in its 2007 report: “In the long term, a sustainable forest management strategy aimed at maintaining or increasing forest carbon stocks, while producing an annual sustained yield of timber, fibre or energy from the forest, will generate the largest sustained mitigation benefit.”[21]

While old-growth forests are often perceived to be in equilibrium, releasing as much carbon dioxide as they capture, or even in a state of decay,[22] studies of soils in undisturbed tropical rain forests, Siberian woods, and in German national parks have found that soils contain enormous amounts of carbon derived from fallen leaves, twigs and buried roots that can bind to soil particles and remain for 1,000 years or more.

A review of numerous scientific studies concluded that forest harvesting has little or no effect on the amount of carbon stored in the soil.[23] As trees grow, they remove carbon from the atmosphere. As they reach maturity, growth slows and ultimately stops as mortality catches up to growth.[24] In most North American forests, this drop happens when a tree is between 60 and 150 years old, depending on the species and environmental factors.[25] Each forest has a different potential to store carbon. For example, this potential is particularly high in the Pacific Northwest where forests are relatively productive, trees live a long time, decomposition is relatively slow, and fires are infrequent. The differences between forests must therefore be taken into consideration when determining how they should be managed to store carbon.[26] Harvesting removes carbon from the forest but some of it is stored in wood products (preventing its immediate release to the atmosphere) and some is available for use as biomass energy (displacing fossil fuel use).[27]


Old-growth forest in the Opal Creek Wilderness, a wilderness area located in the Willamette National Forest in the U.S. state of Oregon, on the border of the Mount Hood National Forest. It has the largest uncut watershed in Oregon.[28]

The increased understanding of forest dynamics in the late 20th Century has led the scientific community to identify a need to inventory, understand, manage and conserve representative examples of old-growth forests with their associated characteristics and values.[29] The literature around old growth and its management is inconclusive about the best way to capture the true essence of an old growth stand. In British Columbia, Canada, old-growth forests must be maintained in each of the province’s ecological units to meet biodiversity needs.[2]

A better understanding of natural systems has resulted in new ideas about forest management, such as managed natural disturbances should be designed to achieve the landscape patterns and habitat conditions that are normally maintained in nature (DeLong 1998; Wong and Iverson 2004). This coarse filter approach to biodiversity conservation recognizes ecological processes and provides for a dynamic distribution of old growth across the landscape.[29] And all seral stages – young, medium and old – support forest biodiversity. Plants and animal's rely on different forest ecosystem stages to meet their habitat needs.[30]

In Australia, the regional forest agreement (RFA) attempted to prevent the clearfelling of defined "Old Growth Forests". This led to struggles over what constitutes "Old Growth". For example in Western Australia, the timber industry tried to limit the area of Old Growth in the karri forests of the Southern Forests Region; this led to the creation of the Western Australian Forests Alliance, the splitting of the Liberal Government of Western Australia and the election of the Gallop Labor Government. Old Growth Forests in this region have now been placed inside National Parks. A small proportion of Old Growth Forest also exists in South-West Australia, and is protected by a Federal laws from logging, which hasn't occurred there for more than twenty years.

Locations of remaining tracts

Wildland fires in U.S. ecosystems. Particularly before 1550, fires happened most frequently in tan, yellow, blue, pink, and light green areas. (Click on image to enlarge)

In 2006 Greenpeace identified that the world's remaining Intact forest landscapes are distributed among the continents as follows:[31]

  • 35% in Latin America. The Amazon rainforest is mainly located in Brazil, which clears a larger area of forest annually than any other country in the world.[32]
  • 28% in North America. North America harvests 10,000 square kilometres of ancient forests every year. Many of the fragmented forests of southern Canada and the US lack adequate animal travel corridors and functioning ecosystems for large mammals.[32]
  • 19% in Northern Asia. Northern Asia is home to the largest boreal forest in the world.[33] The Siberian tiger once roamed across huge areas of Northern Asia but today can only be found in a small area of intact forest near the Sea of Japan. Only about 400 remain in the wild and 800 in zoos.[32]
  • 8% in Africa. Africa has lost most of its intact forest landscapes in the last 30 years. The timber industry is responsible for destroying huge areas of intact forest landscapes and continues to be the single largest threat to these areas.
  • 7% in South Asia Pacific. The Paradise Forests of Asia Pacific are being destroyed faster than any other forest on Earth. Much of the large intact forest landscapes have already been cut down, 72% in Indonesia and 60% in Papua New Guinea.[32]
  • Less than 3% in Europe. In Europe, more than 150 square kilometres of intact forest landscapes are cleared every year and the last areas of the region’s intact forest landscapes in European Russia are shrinking rapidly.[32]

Logging in old growth forests

The large trees in old growth forests are economically valuable, and have been subjected to aggressive logging around the world. This has led to much controversy between logging companies and environmental groups.

In Australia, the regional forest agreement (RFA) attempted to prevent the clearfelling of defined "Old Growth Forests". This led to struggles over what constitutes "Old Growth". For example in Western Australia, the timber industry tried to limit the area of Old Growth in the karri forests of the Southern Forests Region; this led to the creation of the Western Australian Forests Alliance, the splitting of the Liberal Government of Western Australia and the election of the Gallop Labor Government. Old Growth Forests in this region have now been placed inside National Parks. A small proportion of Old Growth Forest also exists in South-West Australia, and is protected by a Federal laws from logging, which hasn't occurred there for more than twenty years.

The island of Tasmania, just off the south east coast of Australia has the largest amount of temperate old-growth rainforest reserves in Australia with approximately 1,239,000 hectares in total[34]. While the local Regional Forest Agreement (RFA) was originally designed to protect much of this natural wealth, many of the RFA old growth forests protected in Tasmania consist of trees of little use to the timber industry. RFA old growth and high conservation value forests that contain species highly desirable to the forestry industry have been poorly reserved. Only 22% of Tasmania’s original tall-eucalypt forests managed by Forestry Tasmania have been reserved. Ten thousand hectares of tall-eucalypt RFA old growth forest have been lost since 1996, predominantly as a result of industrial logging operations. In 2006, approximately 61,000 hectares of tall-eucalypt RFA old growth forests remained unprotected [35]. Recent logging attempts in the Upper Florentine Valley have sparked a series of protests and media attention over the arrests that have taken place in this area. In addition, Gunns Limited the primary forestry contractor in Tasmania has been under recent criticism by political and environmental groups over its practice of woodchipping timber harvested from old growth forests.

See also


Old growth red cedar stump near Port Renfrew, BC
  1. ^ White, David; Lloyd, Thomas (1994). "Defining Old Growth: Implications For Management". Eighth Biennial Southern Silvicultural Research Conference. http://www.treesearch.fs.fed.us/pubs/741. Retrieved 23 November 2009. 
  2. ^ a b Naturally: wood British Columbia’s Forest Diversity
  3. ^ BC Ministry of Forests 2003 Old Growth Forests
  4. ^ Old-growth definitions and management: A literature review BC Journal of Ecosystems and Management
  5. ^ Old Growth Forest Definitions for Ontario Page 17 and Appendix A
  6. ^ Protect the World’s Forests Rainforest Action Network
  7. ^ The world’s remaining great forests. The Guardian 2007
  8. ^ Stamets, Paul (2005). Mycelium Running. Ten Speed Press. pp. 35. ISBN ISBN 1580085792. 
  9. ^ Provincial Non-Spatial Old Growth Order. 2004. Integrated Land Management Bureau, British Columbia, Canada
  10. ^ Forest Stand Dynamics. 1996. Oliver C.;Larson B.
  11. ^ Old growth in the boreal forest: A dynamic perspective at the stand and landscape level Daniel Kneeshaw and Sylvie Gauthier 2003
  12. ^ Frequently Asked Questions nps.gov'.' Retrieved February 11, 2009.
  13. ^ Old growth forest definitions for Ontario page 8
  14. ^ Old-growth definitions and management: A literature review BC Journal of Ecosystems and Management P. 17
  15. ^ Coastal Action Plan page 4
  16. ^ Submissions to XII World Forest Congress 2003 Old-growth forests in Canada – A science perspective
  17. ^ Medicinal Drugs Derived from Rainforest Plants from Mongabay
  18. ^ The Rainforest as a Source For New Pharmaceuticals August, 2008 from Network Science website
  19. ^ IPCC Climate Change 2001: Working Group I: The Scientific Basis
  20. ^ Old-growth definitions and management: A literature review BC Journal of Ecosystems and Management P. 22
  21. ^ Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, 2007 B. Metz, O.R. Davidson, P.R. Bosch, R. Dave, L.A. Meyer (eds) Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA
  22. ^ Old-Growth Forests Can Actually Contribute to Global Warming, Wired magazine, May 19, 2008
  23. ^ [Ter-Mikaelian, M. T., S. J. Colombo, et al. 2008. "Fact and fantasy about forest carbon." Forestry Chronicle 84(2): 166-171]
  24. ^ CORRIM fact sheet 2009 Maximizing Forest Contributions to Carbon Mitigation
  25. ^ Tackle Climate Change – Use Wood Page 17
  26. ^ U.S. Forest Service A Synthesis of the Science on Forests and Carbon for U.S. Forests Page 4
  27. ^ Ecological Society of America A Synthesis of the Science on Forests and Carbon for U.S. Forests Page 4
  28. ^ "Opal Creek Wilderness". The Cranberry House. http://cranberryhouse.biz/Cranberry%20Web%20070502/Opal%20Creek%20Wilderness.html. Retrieved 2007-08-20. 
  29. ^ a b Old-growth definitions and management: A literature review BC Journal of Ecosystems and Management P. 16
  30. ^ BC Ministry of Forests Research Program Seral Stages across forested landscapes 1998
  31. ^ IntactForests.org
  32. ^ a b c d e Intact Forest Landscapes | Greenpeace International
  33. ^ World Boreal Forests: An Introduction
  34. ^ Forest Education http://www.forest-education.com/index.php/tasmania/C233/
  35. ^ Friends of the Earth Australia http://www.foe.org.au/resources/chain-reaction/editions/105/tasmanias-old-growth-forests

External links

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